Development of composite planar fabric in mylonites along the Median Tectonic Line, southwest Japan

Abstract Mylonites along the Median Tectonic Line, southwest Japan commonly contain shear bands comprising S(‐C)‐Ss fabrics. This paper stresses the lithologic control on the orientation, dimension and development of shear bands by comparing the microstructure of the shear bands in different rock types (P mylonites, F mylonites, micaceous phyllonite and quartzose phyllonite). There is no significant change of the α angles (average 21–24°) between Ss and S toward the centre of the shear zone (viz. increasing the intensity of mylonitization) and it is different from the S‐C relationship in a narrow sense. The generation of the composite planar fabric can be classified into four different strain partitioning models: S only type without any slip surface (model A); S‐C type (model B); S‐Ss type with Ss‐slip precedence (model C), and S‐Ss type with S‐slip precedence (model D). Model C is proposed in this paper and is similar to the model for the generation of Riedel shears in brittle shear zones. An unstable slip between porphyroclasts and the matrix during ductile flow can easily initiate shear bands. Formation of a composite planar fabric is initiated according to model A, followed by model C in conditions of increasing strain, and then model D when the angle between S and the shear zone boundary becomes small enough (α/2 = 10°) to produce S‐slip. Thus the generation of the shear bands probably begins in the early stages of shear deformation and continues until the latest stages.